One system, and variations thereof, for cancelling vibrations has been proposed in which the vibration is sensed via an appropriate pick-up, and a cancellation signal is created which is a 180.degree. phase shift of the sensed signal. The cancellation signal is applied to or near the vibration source, thereby cancelling or at least greatly attenuating the vibration source. U.S. patents disclosing such a system are U.S. Pat. Nos. 4,153,815, 4,417,098, 4,489,441, and 4,566,118.
Such systems are based on adaptive signal processing techniques which may result in an inherent instability which could amplify rather than attenuate the vibration. Also, such systems require a direct measurement of a synchronizing signal which provides them directly with the value of the frequency of the excitation source (vibration). Those systems operate on the basis of synthesizing the vibration source signal, synchronizing it with the excitation signal, then delaying such signal to achieve a 180.degree. phase difference and applying it to cancel the effect of the excitation source. This results in a high sensitivity and potentially unstable vibration cancellation due to exact phasing needs.
U.S. Pat. Nos. 4,122,203; 4,153,815; 4,417,098; 4,490,841; 4,527,282; 4,566,118; 4,600,863; and 4,654,871 disclose the work of G. B. Chaplin in the area of repetitive phenomena. The active noise control systems disclosed therein are designed for one-dimensional systems, and all approaches described require some type of "synchronizing" signal. Thus, those systems require some type of sensor linked directly to the excitation source. Those systems cannot be used for random vibration or noise sources. They require a relatively long processing time, and are directed predominantly to acoustic systems, and do not appear to relate to active vibration control.
U.S. Pat. Nos. 4,473,906 and 4,562,589 (Warneka) disclose a departure from the systems disclosed in the aforementioned Chaplin patents, relating to use of a feedforward control signal. The techniques disclosed require a direct measurement from the noise or vibration source, after which this signal is inverted and used to cancel the detected noise or vibration. Using a feedforward signal allows attenuation of random excitations, but these systems again require a direct measurement from the source and in many applications there is no direct access to the noise or vibration source.
U.S. Pat. Nos. 4,667,676 and 4,667,677 disclose an approach based on adaptive filters (LMS and RLMS) and feedback signals to estimate the excitation source signal. The shortcomings of those systems are a) potential for instability, b) a need for a high degree of on-line processing, and c) use of only feedback signals which limits the system's application to broadband noise and vibrations. The disclosed systems have been mostly used in noise control systems.
U.S. Pat. Nos. 4,649,505 and 4,862,506 (Noise Control Technologies) appear to be based on hardware modifications of prior art, using adaptive filters (such as introduced by Widrow; 1960's), and using a LMS algorithm which does not guarantee controlled system stability.
Another system has been disclosed in which a source of vibration is monitored (sensed) and an attenuating signal is applied to or near the source. The attenuating signal is modified in opposition to changes sensed at the vibration source, until the combination of the two results in cancellation of the vibration, or attenuation thereof to some predetermined level. No phase-shifted attenuating signal is employed. Systems of this type are disclosed in a paper entitled "A Multiple Error LMS Algorithm and Its Application to the Active Control of Sound and Vibration" by Stephen J. Elliott, Ian M. Strothers & Philip A. Nelson, IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-33 No. 10, October 1987, and in a paper entitled "A Unified Control Strategy for the Active Reduction of Sound and Vibration" by N. J. Doelman, Journal of Intelligent Material Systems and Structures, Vol. 2 No. 4, pages 558-580, October 1991.
Further, previously proposed vibration control schemes require a fast Fourier Transform (FFT) analyzer, which adds significantly to the cost of the system and/or increases the amount of on-line computation.